The Tunguska black hole

When Jackson and Ryan proposed their impeccably scientific black hole theory, they commented that many attempts had been made to explain the Tunguska event, 'ranging from the prosaic to the bizarre', and then suggested that 'a black hole of substellar mass such as those that have been postulated by Hawking could explain many of the mysteries associated with the event'. Their explanation has never been called 'prosaic' or 'bizarre', but it has certainly been described as 'imaginative and intriguing' by some scientists.

How can a small black hole explain the Tunguska event? Jackson and Ryan's case was based on three main arguments:

High velocity. The researchers assumed that the black hole had the mass of a large asteroid (about 100,000 billion to 10,000,000 billion tonnes), but its geometrical radius could be measured in micrometres. However, its gravitational field could be quite strong for some distance from the body. They also assumed that the black hole's escape velocity - the minimum speed an object must have to free itself from the gravitational pull of a planet or a star - was slightly greater than Earth's escape velocity, which is about 40,000 kilometres per hour. They calculated that, if the black hole began in interstellar space with zero velocity and fell freely to Earth's orbit, its velocity relative to Earth would be between 36,000 and 360,000 kilometres per hour. Thus, the black hole would travel through the last 30 kilometres of the atmosphere in about 1 second.

Bright blue 'tube'. The air around the passing black hole would heat to between 10,000 and 100,000 degrees Celsius. So most of the radiation from the shock front would be ultraviolet rays. The accompanying plasma column would therefore appear blue. 'These results agree well with eyewitness reports of the event and with measurements of the pattern of throwdown of trees at the site', Jackson and Ryan said.

No crater. 'Since the black hole would leave no crater or material residue, it explains the mystery of the Tunguska event', they said. 'It would enter the Earth, and the rigidity of the rock would allow no underground shock wave. Because of its high velocity and because it loses only a fraction of its energy in passing through the Earth, the black hole should very nearly follow a straight line through the Earth, entering at 30 degrees to the horizon and leaving through the North Atlantic.' At the exit point there would be another shock wave and disturbance of the sea surface. Jackson and Ryan suggested that oceanographic and shipping records could be studied to see if any surface or underwater disturbances were observed.

Scientists have found, forgive the pun, many holes in the black hole theory. 'The black hole would have shot straight through the Earth but unfortunately for the theory (although fortunately for us) the exit point, latitude 40 degrees, 50 minutes north, longitude 30 degrees, 40 minutes west, in the mid-Atlantic was not marked by an equally severe shock and blast wave', commented the British astronomer David Hughes in Nature.

Gerald Wick and John Isaacs of the Scripps Institution of Oceanography in California also wrote in Nature: 'Unfortunately, this miniature, hypothetical object cannot account for all the important phenomena known to accompany the event.' Their main argument centred around the small magnetic globules with high nickel content found in the Tunguska region. High nickel content confirms that the globules are of extra-terrestrial origin, but it does not necessarily ensure that they originated in the Tunguska blast.

'Most soil samples collected at random over the globe will contain similar cosmic dust', Wick and Isaacs said. 'The spatial pattern of the globules collected in the Tungus, however, shows that the cosmic dust likely originated from a massive meteorite body of dimensions vastly greater than a few angstroms.' However, they agreed that their discussion did not preclude the possibility that a black hole comprised the nucleus of the comet, or that black holes frequently might be the agents condensing the materials in such bodies.

William Beasley and Brian Tinsley of the University of Texas at Dallas claimed in Nature that several lines of evidence rendered the black hole theory extremely unlikely:

First, many characteristics of the event indicated that the main part of the energy went into an explosion in the air. These characteristics include trees scattered on the ground, without branches or bark, in the direction opposite to the centre of the fall; an intense fire that seared trees; and in ravines, partially protected trees that remained standing, but many with their tops broken. A typical meteorite buries itself below the surface and then dissipates its energy in an underground explosion. The Meteor Crater in

Arizona was produced in this way. 'No significant excavation was caused by the Tunguska explosion', remarked Beasley and Tinsley.

Second, a small cometary nucleus, consisting of a mass of frozen gases mixed with nickel-iron and silicate particles, would have a low degree of cohesion, and would fragment in the air and dissipate most of its kinetic energy before it reached the surface. A small black hole could produce a similar air blast, but would have passed through Earth in 10 to 15 minutes and caused a similar explosion at the point of exit, which would have occurred in the North Atlantic.

Third, about five hours after the Tunguska blast, six microbarographs in England recorded sound waves from the explosion. The approximate distance from the point of impact to the centre of the microbarograph stations is 5,720 kilometres, so the average speed of the waves was about 1,150 kilometres per hour, which is about the usual value for this type of wave. It is clear that the recorded waves were travelling from Siberia, and not from the North Atlantic. Sound waves from the site of the suggested exit explosion should, however, have arrived in England about three hours before the arrival of the Siberian wave. Beasley and Tinsley stressed that they had examined copies of the English microbarograph records, but had been unable to find any sign of waves from the North Atlantic exit point.

Fourth, a thick dust train along the path of the fireball immediately after its passage was noted by eyewitnesses. This observation is consistent with the deposition of the material in the atmosphere, rather than the loss of air into a black hole.

Fifth, exceptionally bright nights in Siberia and Europe imply that extra-terrestrial material was deposited in the upper atmosphere simultaneously with the impact. 'The wide area of atmospheric deposition is comparable to the dimensions of a cometary tail', Beasley and Tinsley said, 'and is not compatible with the idea of slow transport of dust vertically and horizontally from a ground level explosion'. This deposition of dust in the upper atmosphere could give rise to noctilucent clouds, which could account for the bright nights.

'All the evidence favours the idea that the impact which caused the Tunguska catastrophe involved a body with characteristics like a cometary nucleus, rather than a black hole', Beasley and Tinsley concluded.

A further challenge to the black hole theory came from the American scientists Jack Burns, George Greenstein and Kenneth Verosub. In the Monthly Notices of the Royal Astronomical Society they discussed inconsistencies in the predicted and observed thermal changes of soil and rock and seismic activity associated with the event. 'The point of entry of the hole into the Earth should be marked by a patch of melted and resolidified rock of diameter half to four kilometres, overlain by fused soil of comparable extent', they said. 'As the hole entered the soil it would have vaporised the water, oxidised the organic matter and fused the residual material such as quartz, feldspar and mica ... the point of impact should therefore be marked by a depression.' The Southern Swamp, or Kulik's Great Cauldron, is in fact a depression, but they pointed out that 'this depression may predate the Tunguska Event and is not inconsistent with other explanations'.

Burns, Greenstein and Verosub's calculations showed that Jackson and Ryan's black hole would release seismic energy equivalent to 1 million to 100 million megatons of TNT in the Earth, whereas the largest earthquake ever recorded (magnitude 8.3) released the equivalent of only 50 megatons. 'The absence of enormous seismic activity associated with the Tungus event therefore precludes its interpretation as a small black hole', they declared.

A postscript to Jackson and Ryan's popular theory appeared in Rupert Furneaux's book The Tungus Event (1977). Upon learning that no exit pulse had been found on the English microbarographs, the scientists were disappointed at the rejection of their theory: 'It begins to seem that the Tunguska event is more bizarre than any explanation put forward to date.'

Many decades have passed since the publication of Jackson and Ryan's black hole theory. We do now have a better understanding of black holes. Does this new knowledge support their theory? No scientific theory prohibits a wandering black hole striking Earth, but the question is: did a mini black hole pass through Earth on the morning of 30 June 1908?